The relationship between the environment and human activities is becoming better understood. While humans have always depended on the environment for their air, water, and food, only recently have the impact of human activities on the environment become apparent. Modern civilization produces industrial by-products of types and amounts that degrade the environment. Currently, pollution control regulations and technologies are seeking to mitigate these effects.
An important industrial by-product is fly ash. Worldwide, tremendous quantities of coal are burned to generate electricity. Typically, coal is pulverized to a fine powder, pneumatically conveyed into a boiler and burned as a dispersed powder with the heat which is liberated being used to produce steam to power turbines and generate electricity. In the boiler, the carbonaceous constituents in the coal burn and release heat. The non-combustible materials are heated to high temperatures and typically melt and pass through and out of the boiler as fly ash. This fly ash is collected prior to the flue gases going up the stack and being dispersed in the atmosphere. A 1,000 megawatt power plant burns approximately 500 tons of coal per hour. Ash levels in the range of 10% are typical of many coals burned throughout the world. As a result, fly ash is produced at very high volumes throughout the industrialized world.
The economic design of any power plant is necessarily a compromise between capital and operating cost. A factor that has become important in recent years is the air pollution produced by burning coal in large utility power plants. NOx is formed by oxygen and nitrogen reacting at high temperatures. Because NOx formation is favored by high temperatures, one way to reduce NOx emissions is to reduce the temperature in the boiler and reduce excess oxygen. This is typically done through utilizing what are called Low NOx Burners. Many boiler manufacturers produce such Low NOx Burners and many utilities are in the process of installing such devices. An undesired side effect of reducing flame temperature and excess oxygen is an increase in the unburned carbon in the fly ash leaving the boiler.
Another way to reduce NOx emissions is to inject ammonia. This results in selective reactions that reduce the levels of NOx present. The capital cost of ammonia injection is small, and its effects are to some extent additive to those achieved by Low NOx Burners. As a result, many utilities have, and many more will, utilize the injection of ammonia, resulting in ammonia contamination of great volumes of fly ash.
Fly ash can be a useful commodity. The passage of non-combustible minerals present in the coal through the high temperature boiler, followed by quenching in the boiler tube changes the relatively inert clay and shale minerals into glassy ceramic type materials. These glassy inorganic particles react with lime to form cementitious materials. This"pozzolanic" property of fly ash is widely exploited. Fly ash is incorporated into concrete as a substitute for, or addition to, cement where it reacts with free lime liberated during the normal hydration of the cement and produces further property enhancing cementitious materials. Fly ash used in this way produces stronger concrete which is more resistant to environmental attack, resulting in cheaper, higher quality concrete. This valuable use of fly ash as a pozzolan in concrete turns a high volume waste into a high volume useable material.
A further value of using fly ash as a replacement for cement relates to the environmental impact of producing cement. Cement is produced from minerals which are sources of calcium, alumina, and silica. When cement is produced, these minerals are combined in a cement kiln and heated to incipient fusion. For every ton of cement produced, approximately two tons of minerals are mined and approximately one ton of carbon dioxide (CO.sub.2) is emitted into the atmosphere from combustion of required fuels and decomposition of limestone. Replacing cement with fly ash reduces CO.sub.2 emissions on a ton for ton basis.
The use of fly ash in concrete requires that the fly ash have specific physical and chemical properties. The pozzolanic properties are activated in the concrete by the generation of highly alkaline free lime from hydration of the cement. When fly ash contains ammonia, this ammonia is liberated by the action of the highly alkaline solution of the curing concrete. Ammonia is a strong smelling compound that carries the connotation of barnyards, manure and urine. Thus, though the finished properties of the concrete are not adversely affected, the odor is unacceptable to the concrete producer, the contractor working with the concrete and the ultimate concrete user, particularly if the concrete is used in underground or enclosed spaces. See, for example, Van der Brugghen, F. W., Gast, C. H., Van den Berg, J. W., Kuiper, W. H., Visser, R., Problems encountered During the Use of ammonium contaminated Fly Ash.
Fly ash containing less than 100 mg ammonia per kilogram of ash (100 parts per million (ppm)) produces little or no odor when used in the production of concrete. However, the addition of ammonia at the power generation plant typically results in fly ash ammonia contents of 200 to 2500 ppm, rendering the fly ash unacceptable for use in concrete. Thus, reducing air quality problems by controlling the air emissions of power plants increases a solid waste disposal problem and increases CO.sub.2 greenhouse emissions from concrete production. Removal of ammonia from fly ash such that the fly ash can be used in concrete would benefit the utility by avoiding solid waste disposal, the concrete producer by lowering the cost of materials and increasing product quality, and the environment by reduction of emissions of greenhouse gases.
As ammonia use by utilities has increased, processes have been suggested to remove ammonia from fly ash. These processes range from heating the ash and driving off the ammonia by thermally decomposing the ammonium salts present, to chemical treatment of the ammonium salts to release ammonia gas. A thermal decomposition process is described by Fisher, Blackstock, and Hauke in Fly ash benefication using an ammonia stripping process. 12.sup.th Int. Symposium on Coal Combustion By-Products Management and Use, January 1997, 65-1 to 65-8. Thermal decomposition requires substantial energy use since the decomposition temperature of the most probable ammonium salts present, ammonium sulfate and ammonium bisulfate, are 808.degree. F. and 813.degree. F. respectively. The energy needed to reach and maintain these ash temperatures, and process equipment required to flush the ammonia gas from the mass of the ash, separate the ash from the ammonia gas, and cool the ash, make this an expensive option.
Chemical treatment of the fly ash to decompose the ammonium salts to ammonia gas takes advantage of the same chemical processes by which ammonia gas is released when ash containing ammonia is used in concrete production. Perhaps the most developed and the most conventional process is described in a European Patent No. 0135148 ('148) to Huller, Wirsching, and Hamm. In the process disclosed in the '148 patent, the ammonia containing fly ash is mixed with lime and water and the mixture is allowed to sit for 1/2 to 2 hours while ammonia is evolved. This process calls for particular attention to be paid to not allowing the temperature of the ash-lime-water mixture to fall below 100.degree. C. Temperature control is achieved by controlling the amount of lime added and ratio of lime to water, so that the heat of hydration of the lime is sufficient to evaporate all of the water added and maintain the temperature of the mixture mass above 100.degree. C. This results in a free flowing ash containing less than 10 ppm ammonia.
The process of the '148 patent, while producing an ash of acceptably low ammonia content, presents a number of difficulties. The time for the reaction is inconveniently long, necessitating large equipment. The quantity of lime required, from 5 to 30% of the mass of the fly ash, is quite high and will change the resulting usefulness of the ash in concrete. By present ASTM specifications covering fly ash for use in concrete (ASTM C618-97), the sum of the alumina+silica+iron content must be 70% or greater for Class F fly ash and 50% or greater for Class C fly ash. Addition of lime at the levels suggested in this process would frequently result in a Class F ash to be reclassified as a Class C ash and a Class C ash to no longer meet the minimum requirements for use in concrete. The presence of substantial quantities of free lime in the processed fly ash would also change its behavior in concrete, decreasing its usefulness. Thus for economic factors and changes in the chemistry of the resulting ash, this process appears unsuitable for reducing the ammonia content of fly ash to be used in concrete.
U.S. Pat. No. 5,069,720 ('720) to Epperly and Sprague describes a process by which the emission of ammonia from ammoniated fly ash can be reduced by, among other processes, treatment of fly ash with lime and water. The described processes are directed to forming a physical barrier to the flow of water to the fly ash and the flow of gaseous ammonia from the fly ash. Relatively large quantities of lime are again used: 10% by mass of the ash. In particular, the described process of the '720 patent is for treating the fly ash for disposal purposes, and for preventing ammonia from being emitted to the air, rather than aiding in removal from the ash. The use of lime is rejected as being less effective than the other compositions disclosed in the patent.
U.S. Pat. No. 5,211,926 ('926) to Martin, et al. describes a process for rendering ammonia contaminated fly ash usable in concrete or other applications. The process described uses large amounts of water, in the range of 25 to 40%. The water is mixed with the ash in batches and the resulting ammonia drawn off by vacuum. Adjustment of the material pH to greater than 10 is identified as particularly effective. No specific means to accomplish this pH adjustment is given. However, a large majority of the water must be removed to produce a marketable product; since the ASTM specifications require a maximum water content of 3%.